Your guide to Tyre Pressure Monitoring Sytems (TPMS)

Posted on December 2nd, 2013 by GEM Motoring Assist

Tyre Pressure Monitoring Systems may fulfil a valuable safety requirement but, as so few people are familiar with the technology, Rob Marshall demystifies their inner workings.

Type pressure monitor

Tyre Pressure Monitoring Systems (TPMS) were adopted initially in the mid-1980s but they have become more popular within the past decade, with many different manufacturers embracing the technology. As the correct tyre pressure is crucial from both safety and environmental standpoints, fitting Tyre Pressure Monitoring Systems is now being considered as mandatory for all new vehicles.

Tyre pressue warning

On some cars, a warning graphic illuminates, should a puncture be detected.

HOW PRESSURE MONITORS WORK

Indirect types work by using an existing vehicle sensor, usually that of the ABS (anti-lock brake) system, to detect low pressures. Unfortunately, indirect Tyre Pressure Monitoring Systems are often not very accurate and require the driver to reset the system every time a tyre is inflated.

Direct Tyre Pressure Monitors are more common and effective. They consist of a combined tyre pressure sensor and valve unit, attached to the inside of each wheel rim.

Pressure sensor

Tyre Pressure Sensor, fitted to a road wheel (tyre removed)

 The sensors emit a low-frequency radio signal to a receiver, the message from which is sent to the vehicle’s onboard computer. The driver is then alerted if the tyre pressures fall below the prescribed limits.

As getting power to a rotating wheel is very difficult, most tyre pressure sensors are equipped with an internal battery, which is intended to last approximately ten years. Once it is exhausted, the entire tyre pressure sensor and combined valve must be replaced.

COMMON FAULTS WITH TYRE PRESSURE MONITORS

Despite their more widespread use in recent years, it seems that few aftermarket tyre fitters are familiar with Tyre Pressure Monitoring technology. As the traditional Schrader tyre valve has to be renewed on conventional rims, before mounting a replacement tyre, many technicians assume, incorrectly, that the entire tyre pressure sensor should be replaced. This can add to the cost of replacing a single tyre by at least 50%. Instead, a service kit can be bought for many types of tyre pressure sensor, which should include the valve core that is located within the valve stem.

Type pressure
Have the valve core, inside the sensor, renewed at every tyre change.

 Tyre pressure

Ensure that the tyre fitter knows that pressure sensors are fitted to your car, as they can be damaged easily.

Tyre pressure

Should it need replacement, removing a sensor is not a difficult job, with the tyre removed.

Other than flat batteries, Tyre Pressure Monitor Systems can develop internal faults that can alert the driver to a potential deflation, when there is none. Yet, replacing a faulty sensor might be only part of the solution. On most cars, the new tyre pressure sensor requires activating and then coding to the vehicle’s on-board computer. It appears that most aftermarket tyre fitting cannot offer this service and so a main dealership visit might be the only answer.

 Tyre pressure sensors

Many new sensors require triggering, with the use of a special electronic tool. Pictured is a wireless ‘exciter’ being used to activate the sensor.

Tyre pressure

The car’s computer hardware also needs coding, with diagnostic equipment, to recognise the new sensors.

The final ‘repair’ option is to disable the Tyre Pressure Monitor system completely. This can be performed on most makes and models but the procedure will require specialist diagnostic equipment, which is often the preserve of single-make specialists and main franchise dealerships. This option might be the cheapest one but it does carry a safety risk, in that a driver will no longer be alerted in the event of low pressure or a sudden tyre deflation.